The invention relates generally to the design of tubular reactors for carrying out exothermic chemical reactions. In a preferred mode, it relates to the process for catalytically oxidizing ethylene to ethylene oxide and reactors useful therein.
Generally, the ethylene oxidation reaction is carried out in multi-tube reactors in which the catalyst is disposed inside the tubes and the exothermic heat of reaction is removed by a fluid circulating on the outside of the tubes. The gases fed to the catalyst tubes contain ethylene and oxygen, along with other gases such as nitrogen, carbon oxides, and argon. The conditions under which the reaction takes place will affect the detailed design of the equipment, but they do not critically influence the effectiveness of the present invention.
In commercial reactors, the gases leave the reaction tubes at a higher temperature than the coolant on the shell side. The temperature of the gases from each tube will depend upon the heat released during the oxidation and the amount of heat removed by the coolant. Operating conditions will be adjusted to produce the best results. As the catalyst declines in activity the outlet temperature is increased.
The prior art has been concerned with the methods by which the effluent gases are cooled after the reaction. At the outlet of the catalyst bed, the gas temperature may be about 230.degree. to 300.degree. C., depending upon various factors. At these temperatures, it is important to cool the gases promptly to minimize the loss of ethylene oxide by isomerization to acetaldehyde and, particularly as the temperature increases, to avoid burning of the hydrocarbons to carbon oxides and water. The burning may take place in a rapid and localized manner causing excessive pressures and temperatures, thus forcing corrective actions to be taken which cause loss of ethylene oxide production and in extreme cases endangering the equipment. Thus, if the temperature of the effluent gases can be lowered quickly, operation at the most efficient conditions is made safer and more stable.
Cooling a large number of tubes uniformly has been found to be difficult. In addition, the heat removed must be usefully employed if the most efficient operation is to be obtained.
In British Patents Nos. 1,449,091 and 1,449,092, it is shown to be typical to exchange heat between the effluent gases and the incoming feed gases in an external heat exchanger. The disadvantages of such designs are discussed in connection with disclosure of the patentees design in which the effluent gases provide heat to the feed gases via a closed heat exchange loop. The feed gases are heated in a contiguous section of the reactor tubes which contain an inert packing to facilitate heat transfer. The effluent gases may be sent to an external heat exchanger, which preferably contains no solid packing, or may give up heat and cool to below 150.degree. C. in a second contiguous section of the tubes, which it is said, may be either packed or not. Actually, use of packing in this section is considered likely since heat transfer in empty tubes is inefficient as will be seen.
As pointed out in U.S. Pat. No. 4,061,659 use of packing in the cooling zone is desirable since it reduces the residence time at high temperatures and consequently reduces the loss of ethylene oxide. The patentee stated that it is important to minimize the surface area of the inert packing to limit losses of ethylene oxide. This appears consistent with earlier patents which suggested such high-surface area solids as alumina and silica be used to isomerize alkylene oxides (see U.S. Pat. No. 2,660,609). Another reason for using inert packing in these tubes is to maintain the high velocities and turbulence needed for effective heat removal. Without packing, the effluent gases would slow down and the tube-side heat transfer coefficient would be reduced by about 80-90.degree. %, thereby drastically reducing cooling of the reaction gases. This would require additional tube length and undesirable residence time to provide the needed cooling.
The use of packing in contiguous cooling sections is shown also in Japanese Published Applications No. 32408/79 and 19206/80.
Obtaining uniform distribution of the cooling fluid is important if each tube is to be cooled to substantially the same temperature. While the bulk temperature of the effluent gases may be lowered adequately, it is undesirable for some tubes to be cooled below the bulk temperature, while others remain at too high a temperature. Ideally, each tube should be cooled equally by a stream of cooling fluid having the same temperature. The present invention relates to that objective.